Liquid-phase diffusion bonding alloy foils for joining heat-resistant metals in oxidizing atmospheres

ABSTRACT

Alloy foils for liquid-phase diffusion bonding of heat-resisting metals in an oxidizing atmosphere comprise 6.0 to 15.0 percent silicon, 0.1 to 2.0 percent manganese, 0.50 to 30.0 percent chromium, 0.10 to 5.0 percent molybdenum, 0.50 to 10.0 percent vanadium, 0.02 to 1.0 percent niobium, 0.10 to 5.0 percent tungsten, 0.01 to 0.5 percent nitrogen, 0.10 to 5.0 percent boron, plus 0.005 to 1.0 percent carbon, and/or either or both of 0.01 to 5.0 percent titanium and 0.01 to 5.0 percent zirconium, all by mass, with the balance comprising nickel and impurities, and have a thickness of 3.0 to 300 μm. Alloy foils for liquid-phase diffusion bonding of heat-resisting metals in an oxidizing atmosphere are also available with substantially vitreous structures.

TECHNICAL FIELD

This invention relates to liquid-phase diffusion bonding of metals, andmore specifically to liquid-phase diffusion bonding materials suited forthe liquid-phase diffusion bonding of heat-resisting steels and alloysteels or between heat-resisting alloy steels and carbon steels inoxidizing atmospheres that form joints having high bonding strength andgood creep characteristics at high temperatures.

BACKGROUND OF THE INVENTION

Liquid-phase diffusion bonding is carried out by inserting between thematerials to be joined an alloy having a melting point lower than thatof the materials to be joined and an eutectic composition in the form offoil, powder or plated coating. Bonding is achieved by applying pressureand heating to a temperature just above the liquidus line of theinserted alloy (hereinafter called the inserted metal). This is a kindof solid-phase bonding in which the bond is completed by melting andisothermal solidification.

Liquid-phase diffusion bonding is used where residual stress anddeformation must be avoided as far as possible and also for joininghigh-alloy and heat-resisting steels that are difficult to weld.

The metals to be bonded by liquid-phase diffusion bonding often contain0.50 percent or more chromium. Chromium-bearing steels generally havehigh oxidation and corrosion resistance because of the coherent films ofchromium oxide (mostly as Cr₂ O₃) formed on the surface. The heatapplied for bonding forms oxide films on the surface of the bond whichinhibit the wetting of the inserted metal and significantly prevent thediffusion of atoms required for the completion of the bond.

It has therefore been necessary to carry out liquid-phase diffusionbonding in a vacuum or in an inert or a reducing atmosphere, asdisclosed in Japanese Provisional Patent Publications Nos. 81458 of1978, 34685 of 1987 and 227595 of 1987. This requirement has greatlyincreased the bonding cost.

By making many studies, the inventors found that liquid-phase diffusionbonding can be carried out in oxidizing atmospheres if the insertedmetal contains vanadium as described later. Although vanadium raises themelting point of the inserted metal, it was also found that thevanadium-bearing inserted metal can have an excellent bondability whenother elements (such as silicon that is controlled in this invention)are properly controlled.

Practically no vanadium-bearing alloy foils, which increased siliconcontents, for liquid-phase diffusion bonding have been proposed. U.S.Pat. No. 3,856,513 discloses alloys having a composition MaYbZc. M is ametal selected from the group comprising iron, nickel, cobalt, vanadiumand chromium, Y is an element selected from the group comprisingphosphorus, boron and carbon, Z is an element selected from the groupcomprising aluminum, silicon, tin, germanium, indium, antimony andberyllium, a is in the range of approximately 60 to 90 atomic percent, bis in the range of approximately 10 to 30 atomic percent, and c is inthe range of approximately 0.1 to 15 atomic percent. Alloys of this typehave been manufactured on an industrial scale by rapidly cooling moltenproducts with conventional technologies.

Being amorphous, with vanadium used as the base component, these alloysare not intended as alloy foils for bonding. Besides, it is extremelydifficult to achieve liquid-phase diffusion bonding with this type ofalloys whose silicon contents are low and melting points areconsiderably higher than the foils according to this invention. Theboron content of this alloy is entirely different from that of the foilsaccording to this invention. Because of the high boron content, coarseprecipitates are formed in the molybdenum- or chromium-bearing metalnear the bond. The resulting bond has a much lower strength than thebonds formed by the use of the foils according to this invention.Japanese Provisional Patent Publication No.81458 of 1978 discloses thefoil of the alloy according to U.S. Pat. No. 3,856,513. Containing novanadium at all, however, this foil does not permit liquid-phasediffusion bonding in an oxidizing atmosphere.

Based upon the above knowledge, the inventors discovered thatliquid-phase diffusion bonding in oxidizing atmospheres is possible byusing an inserted metal containing 0.1 to 20.0 atomic percent vanadium,with the addition of silicon. The technology to provide the alloy foilssuited for liquid-phase diffusion bonding in oxidizing atmospheresdisclosed in Japanese Provisional Patent Publication No. 151378 of 1990is based on the above discovery. This alloy contains 0.5 to less than10.0 atomic percent boron, 15.0 to 30.0 atomic percent silicon, 0.1 to20.0 atomic percent vanadium, one or two elements selected from group(A) comprising 0.1 to 20.0 atomic percent chromium, 0.1 to 20.0 atomicpercent iron and 0.1 to 20.0 atomic percent molybdenum, and/or one ortwo elements selected from the group (B) comprising 0.1 to 10.0 atomicpercent tungsten and 0.1 to 10.0 atomic percent cobalt, with theremainder essentially comprising nickel and inherent impurities. Thisalloy is substantially vitreous and provided in the form of foils havinga thickness of 3.0 to 120 μm.

Because of the increasing straining global energy and environmentalproblems, power plants today are operating under much severer conditionsthan before. Especially the temperature and pressure of operating steamare so high that commercially marketed conventional steels are no longersuited for use with extra-super critical pressure power plants that willbe constructed in the future.

The creep strength of heat-resisting steels is governed bysolid-solution strengthening when aging time is short and byprecipitation strengthening when aging time is long. The solid-solutionstrengthening elements forming solid solutions in steel firstprecipitate as M₂₃ C₆ and other stable carbides as a result of aging.When aging time is longer, the precipitates coalesce and coarsen, thuslowering the creep strength. To maintain the creep strength ofheat-resisting steels at a high level, many studies have been made aboutmethods for keeping the solid-solution strengthening elements in them inthe form of solid solutions for a long time, without causingprecipitation.

For example, Japanese Provisional Patent Publications Nos. 89644 of1988, 231139 of 1986 and 297435 of 1987 disclose that the use oftungsten as a solid-solution strengthening element permits theattainment of ferritic heat-resisting steels having much higher creepstrength than the conventional molybdenum-added ferritic heat-resistingsteels. Many of these steels are single phase of tempered martensite.With the advantage of ferritic steels having good resistance tooxidation by steam combined with high strength, these steels areexpected to find use as the next-generation materials forhigh-temperature and high-pressure service environments.

Because of their increased high-temperature strength, however, theheat-resisting steels just described are difficult to weld. Especiallythe weld metal and welded bond have a tendency to become harder. Some ofsuch ferritic heat-resisting steels do not satisfy safety standardsunless long post-welding heat treatment is applied.

Some boiler plants recently planned use urban wastes and other mattersor chemical substances as fuels. The boilers to be used in such highlycorrosive environments require pipes of nickel-base alloys or doublepipes consisting of one pipe of these alloys and the other pipe of newferritic heat-resisting steels mentioned earlier. However, there hasbeen no effective ways to join together pipes of these highcorrosion-resistance steels. The conventionally available has beencostly because it comprises the steps of depositing large quantities ofvery expensive nickel-base alloys by simple buildup welding, applyingheat treatment to the entire length of the steel pipe, forming a groovefor welding, and then applying welding.

Effective industrial technologies to join together such difficultlyweldable materials have been long awaited and enormous amounts of timeand money have been spent in their studies.

One of the most promising solutions is the liquid-phase diffusionbonding methods including the one according to this invention.Liquid-phase diffusion bonding has been applied for trials to, forexample, rocket engine nozzles and space-craft landing gears of titaniumalloys used in high-temperature environments and requiring adequatehigh-temperature strength and a high degree of reliability. Still,liquid-phase diffusion bonding has not been used in the construction ofpower plants.

Particularly, the conventional nickel-base alloy foils for liquid-phasediffusion bonding do not provide uniform mechanical properties when itis used for bonding new high-strength heat-resisting steels containingtungsten because of the great difference in alloy constituentstherebetween. Furthermore, it cannot be carried out in an oxidizingatmosphere. As is obvious from their chemical compositions, it isimpossible to achieve sound bonding at low cost.

This invention provides a solution for the problems that have beenimpossible for the conventional approaches to solve. More specifically,this invention provides a new liquid-phase diffusion bonding technologyfor joining heat-resisting steels and heat-resisting steel pipes used inhigh-temperature environments. The object of this invention is toprovide alloy foils for liquid-phase diffusion bonding that permitproducing bonded joints having a high degree of reliability and heatresistance by carrying out liquid-phase diffusion bonding in oxidizingatmospheres.

SUMMARY OF THE INVENTION

The object of this invention that is based on the above discovery is toprovide liquid-phase diffusion bonding alloy foils for joiningheat-resisting metals in oxidizing atmospheres comprising 6.0 to 15.0mass percent silicon, 0.1 to 2.0 mass percent manganese, 0.50 to 30.0mass percent chromium, 0.10 to 5.0 mass percent molybdenum, 0.50 to 10.0mass percent vanadium, 0.02 to 1.0 mass percent niobium, 0.10 to 5.0mass percent tungsten, 0.01 to 0.5 mass percent nitrogen and 0.10 to 5.0mass percent boron, with the remainder comprising nickel and impuritiesand having a thickness of 3.0 to 300 μm or liquid-phase diffusionbonding alloy foils for joining heat-resisting metals in oxidizingatmospheres having the same composition and having a substantiallyvitreous nature.

The liquid-phase diffusion bonding alloy foils for joiningheat-resisting metals described above may also contain, as required,0.005 to 1.0 mass percent carbon and either or both of 0.01 to 5.0 masspercent titanium and 0.01 to 5.0 mass percent zirconium.

The term "oxidizing atmospheres" as used in this invention means theatmospheres containing not less than 0.1 volume percent oxygen gas andhaving a partial pressure of oxygen of not lower than 10³ atm. Even areducing gas, such as one containing hydrogen gas, hydrogen sulfide orwater vapor, is an oxidizing atmosphere if its oxidizing power isequivalent to an oxygen concentration of not less than 0.1 percent.

The term "melting point" means the solidus line in the equilibriumdiagram of alloys composed of two or more principal metallic components,unless otherwise stated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing an example of a bondedproduct.

FIG. 2 shows how a liquid-phase diffusion bonded clad sheet steel ismade.

FIG. 3 shows a completed clad sheet steel and how a tensile specimen forevaluating the fracture strength of the bonded joint.

FIG. 4 shows the relationship between the concentration of vanadium inthe inserted metal and the fracture strength of the bonded joint.

FIG. 5 shows the relationship between the concentration of silicon inthe inserted metal and the fracture strength of the bonded joint.

FIG. 6 shows the relationship between the concentration of boron and thefracture strength of the bonded joint.

FIG. 7 shows the influence of the thickness of the inserted metal on thetime required for obtaining a bonded joint having a fracture strength of40 kg/mm² or above by liquid-phase diffusion bonding.

FIG. 8 shows the thickness D of the oxide film formed by heating at 700°C. for 1000 hours that serves as an index of the resistance of thealloys according to this invention to oxidation at high temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a detailed description of this invention.

First, the reason for limiting the contents of alloying constituents asdescribed before is given below.

Silicon is an element that effectively lowers the melting point ofnickel that constitutes the matrix of the alloys according to thisinvention. The alloy foils according to this invention have relativelyhigh melting points because of the characteristically high contents ofvanadium. To avoid the resulting lengthening of the bonding time, notless than 6.0 percent silicon must be added. If over 15.0 percentsilicon is added, however, coarse oxides containing silicon are formedin the inserted metal when liquid-phase diffusion bonding is carried outin oxidizing atmospheres, thereby impairing the strength and toughnessof the bonded joint. Hence, silicon content is limited between 6.0 and15.0 percent.

Manganese increases strength by forming a solid solution in the nickelmatrix. Manganese content is limited between 0.1 and 2.0 percent becausethe strengthening effect is unobtainable when the content is below 0.1percent and excessive strengthening and impairment of bond toughnessoccur over 2 percent.

Chromium is an extremely important element that enhances the corrosionand oxidation resistance of steels and alloys used in high-temperatureenvironments and increases the hardenability of steels. For the bondedjoint to acquire sufficient corrosion resistance and harden-ability,chromium content of at least 0.50 percent is necessary. Chromiumaddition in excess of 30.0 percent markedly raises the melting point ofalloy foils, impairs workability, and raises the liquid-phase diffusionbonding temperature to above 1400° C. that is beyond the practicallimit. Therefore, chromium content is limited between 0.50 and 30.0percent.

Like chromium, molybdenum is also essential for the improvement of thecorrosion resistance of bonded joints. Molybdenum is particularlyeffective in increasing resistance to stress corrosion cracking.Molybdenum also increases the desirable effect of the alloy foilsaccording to this invention as it increases the creep strength of bondedjoints at high temperatures by solid solution strengthening. Molybdenumaddition under 0.10 percent does not produce the desired effect, whereasmolybdenum, in conjunction with boron which is the diffusing element inthe alloy foils of this invention, precipitate Fe₂ MoB₂ or Fe₂ MoB₄ typehigh-melting point borides at grain boundaries. Thus, molybdenum contentis limited between 0.10 and 5.0 percent.

Vanadium increases the wettability between chromium-bearing alloys andthe molten inserted metal by melting and spheroidizing the chromiumoxide film at the surface thereof, thus preventing the chromium oxidefilm from inhibiting the diffusion of boron. As such, vanadium isextremely important for the achievement of liquid-phase diffusionbonding in oxidizing atmospheres. Vanadium addition under 0.50 percentis insufficient for the melting of chromium oxide film to take place,whereas addition in excess of 10.0 percent raises the melting point ofthe inserted metal to above 1300° C. and, thus, makes it practicallyimpossible to achieve liquid-phase diffusion bonding. Hence, vanadiumcontent is limited between 0.50 and 10.0 percent.

Niobium forming fine precipitates in the matrix as carbides, nitrides orcarbonitrides is particularly effective in increasing thehigh-temperature creep strength of bonded joints. Addition under 0.02percent produces little effect because niobium is lost as oxides whenthe surface of the bonded material is oxidized. When added in excess of1.0 percent, niobium causes intercrystalline segregation and embrittlesbonded joints. Therefore, niobium content is limited between 0.02 and1.0 percent.

Tungsten markedly increases the high-temperature creep strength ofbonded materials by solid-solution strengthening. Thus, tungsten isindispensable for balancing the mechanical properties of recentlydeveloped heat-resisting materials having high creep strength and bondedjoints. While addition under 0.10 percent produces no desired effect,addition in excess of 5.0 percent lowers the high-temperature strengthby precipitating coarse Laves phases as a result of interdendriticsegregation. Accordingly, tungsten content is limited between 0.10 and5.0 percent.

Nitrogen greatly increases high-temperature strength by forming fineprecipitates of carbides, nitrides or carbonitrides in the matrix, inconjunction with vanadium. Addition under 0.01 percent does not producesufficient effect, whereas nitrogen forms boron nitrides with boron,which is the main diffusing element of the alloy foils according to thisinvention, when added in excess of 0.5 percent, with the resultingcoarse precipitates impairing the toughness of bonded joints. Thus,nitrogen content is limited between 0.01 and 0.5 percent.

Boron is a diffusing element necessary for the attainment of isothermalsolidification that is, in turn, necessary for the achievement ofliquid-phase diffusing bonding. Boron is also necessary for lowering themelting point of nickel matrix to below that of the bonded material. Toachieve the above objects, a minimum of 0.10 percent boron should beadded. However, the inventors found that boron forms coarse borideslarger than 5 μm at grain boundaries of molybdenum- and chromium-bearingalloys near the bonded joint, thereby greatly lowering the strength ofthe bonded joint, when added in excess of 5.0 percent. Hence, boroncontent is limited between 0.10 and 5.0 percent. The low boron contentis one of the characteristic features of this invention.

In addition to the basic constituents described above, the alloy foilsaccording to this invention may also contain one or two elementsselected from (A) 0.005 to 1.0 percent carbon and/or (B) 0.01 to 5.0percent titanium, and 0.01 to 5.0 percent zirconium.

Carbon forms carbides. More specifically, carbon forms stable carbidesin conjunction with niobium, vanadium, tungsten, molybdenum, chromium,titanium, zirconium and some other elements. When proper heat treatmentis applied, carbon is finely dispersed in steel, thereby increasing thecreep rupture strength of bonded joints. At the same time, carbonincreases the strength of metallic materials as interstitial atomstherein that form interstitial solid solutions. Therefore, carboneffectively balances the mechanical properties of bonded joints to thoseof the high-strength materials to be bonded together. Under 0.005percent, however, carbon does not exhibit sufficient strength increasingeffect. On the other hand, carbon addition in excess of 1.0 percentcoarsens carbides to such an extent as to inhibit the enhancement ofcreep strength. Therefore, carbon content is limited between 0.005 and1.0 percent.

Titanium and zirconium trap and fix oxygen atoms infiltrating into themetal at the bonded joint when liquid-phase diffusion bonding is done inoxidizing atmospheres, thus forming titanium oxides (Ti_(x) O_(y)) andzirconium oxide (ZrO₂). Removed as slag during upsetting, titanium andzirconium oxides have an effect to clean the bonded joint. When added inlarger quantities, titanium combines with carbon to form fineprecipitates of carbides, thus increasing the high-temperature creepstrength of bonded joints. Addition under 0.01 percent does not producethe desired effect, whereas addition in excess of 5.0 percent causessegregation along grain boundaries that markedly embrittles bondedjoints. Therefore, titanium and zirconium contents are limited between0.01 and 5.0 percent. The alloying elements described above may be addedeither singly or in combination.

The vitreous crystalline structure of alloy foils according to thisinvention assures uniform melting required for the achievement ofsatisfactory liquid-phase diffusion bonding. If alloy foil has anonuniform structure with segregation of alloying elements, homogeneousbond interface is unobtainable because the melting point of the insertedmetal varies from point to point. If homogeneous structure and alloyfoil are readily obtainable, the crystalline structure need not bevitreous.

The alloy foils for liquid-phase diffusion bonding according to thisinvention can be provided in various forms of inserted metal. Forexample, it is not only entirely possible but also most appropriate toform alloys having the compositions specified in claims 1 to 4 intofoils by quenching. There are several variations of liquid quenchingthat is basically accomplished by projecting molten alloy through anozzle onto a cooled plate that cools and solidifies the molten alloy bythermal contact. Of these variations, a process known as the single-rollprocess is well suited. Centrifugal quenching using the inner wall of adrum, a process employing an endless belt and their improvements, suchas those including an auxiliary roll or a roll surface temperaturecontrol unit, or casting under reduced pressure, in vacuum or in inertgases are also applicable. A dual roll process achieves quenching andsolidification by pouring molten alloy between paired rolls.

The alloy foils according to this invention can also be prepared bycombining vacuum melting, casting, ordinary rolling and annealing.

Thinner alloy foils are better suited for liquid-phase diffusionannealing because mechanical properties vary less near the bonded jointand bonding is accomplished in shorter time. When the thickness is under3 μm, however, the absolute quantity of vanadium is insufficient formaking harmless the chromium oxide film at the surface of the bondedmetal. When the thickness exceeds 300.0 μm, on the other hand, more than10 hours are required to complete liquid-phase diffusion bonding.Therefore, foil thickness is limited between 3.0 and 300.0 μm.

This invention relates to alloy foils for liquid-phase diffusionbonding. Because bonding can be carried out in the atmosphere, the alloyfoils according to this invention can also be used for brazing,soldering and other metal joining processes.

Example!

Table 1 shows specimens of approximately 100 g of alloys having thecompositions specified in claims 1 to 4 which were formed into 2 to 215mm wide and 50.0 μm thick foils by (1) quenching by the single rollprocess (using a copper alloy roll with a diameter of 300 mm) or (2) hotrolling by a conventional method alloys melted in a vacuum meltingfurnace and cast. The foils hot-rolled by the method (2) were subjectedto homogenizing annealing at 700° C. for 10 hours to eliminatemacroscopic compositional inhomogeneities. The foils quenched by themethod (1) were cast by maintaining the peripheral roll speed between5.0 and 15.0 m per second. The methods employed are shown in Table 1.The methods (1) and (2) are respectively designated by 1 and 2 in thecolumn headed "Method". The width and thickness of the obtained foilswere checked at five points. Table 1 also shows the melting pointsdetermined by means of a differential thermoanalyzer (DTA).

Table 1 shows the chemical compositions of alloy foils in mass percentdetermined by chemical analysis. All alloy foils have nickel-basematrices. The balance between 100 percent and the total percentage ofother components is the total content of nickel and inherent impurities.The foils prepared under the conditions described before have amorphous,crystalline and partially amorphous and crystalline structures. The typeof crystal structures depend upon the chemical composition of alloyfoils.

Table 2 shows the chemical compositions and other properties of alloyfoils prepared for the purpose of comparison with the alloy foils ofthis invention.

Liquid-phase diffusion bonding was carried out using the inserted metalssatisfying the requirements specified in claims 1 to 5, listed in Table1, and the other inserted metals prepared for the purpose of comparison,including those of the conventional types, listed in Table 2, totalingsixty-five in number.

All alloy foils listed in Table 2 have nickel-base matrices. The balancebetween 100 percent and the total percentage of other components is thetotal content of nickel and inherent impurities. The alloy foils inTable 2 were also prepared by the same methods as those used in thepreparation of the alloy foils according to this invention listed inTable 1.

The test specimens of chromium-bearing ferritic and austeniticheat-resisting steels and 62 percent nickel-base high heat-resistingsteel, measuring 100 by 1000 by 2000 mm, in thickness, width and length,as shown in FIG. 1. An inserted metal was placed between two plates asshown in FIG. 2. Table 3 shows the chemical compositions of the metalsjoined together by liquid-phase diffusion bonding. Reference numerals 1and 2 in FIG. 2 designate the metals (carbon and alloy steels) joinedtogether, whereas reference numeral 3 denote the alloy foil (insertedmetal) for liquid-phase diffusion bonding. The thickness of the insertedmetal was 3.0 to 500 μm. Bonding was effected in the atmosphere using alarge heating furnace at 1050° to 1300° C. which were between just abovethe melting points of the alloy foils and the melting point plus 50° C.Bonding was carried out under gravity, without applying any extrapressure, and in one hour. To secure the strength, corrosion resistanceand toughness of the bonded metals, annealing, combinations of hardeningand annealing, annealing and tempering, and hardening and annealing andtempering were applied singly and in combination. During these heattreatments, interdiffusion of elements between the bonded metalsproceeded to achieve the homogenization of the bonded joints. Because ofthe low boron contents in the inserted metals, however, hardly anyformation, increase or growth of precipitates occurred.

Next, the soundness of the bonded joints was checked by the methodaccording to JIS G 0601-5, "Method of Ultrasonic Flaw Detection for CladSteel Sheets". In all specimens with the inserted metals satisfying therequirements of claims 1 to 5, the bonded area ratio was 0 percent.

Then, the tensile test specimens of JIS A-2 type with the axis extendingin the direction of thickness were taken to determine the relativefracture strength of the bonded joint at room temperature using anInstron tension tester.

Tensile fracture strength of bonded joints depend upon the propertiesand thickness of the bonded metals, service environments, and otherfactors. For the purpose of the test, 40 kg/mm² was assumed to be aminimum necessary strength. The bonded joints whose fracture strengthexceeded 40 kg/mm² were considered to be satisfactory. Test results arealso shown in Table 1.

FIG. 4 shows the influence of the concentration of vanadium in theinserted metal on the fracture strength of the bonded joint. When thevanadium content was under 0.5 mass percent, the fracture strength ofthe bonded joint was low because the detrimental effect of the chromiumoxide film at the surface of the bonded metal was not sufficientlyremoved. Vanadium content of 0.5 percent or above proved to make thefracture strength of the bonded joint equal to or above the level of thebonded metal and effectively remove the detrimental effect of thechromium oxide film. Vanadium content in excess of 10.0 percent,however, proved to raise the melting point of the inserted metal,thereby shortening the time required for bonding and lowering thefracture strength of the bonded joint.

FIG. 5 similarly shows the relationship between silicon content and thefracture strength of bonded joints. The fracture strength of bondedjoints was low when silicon content was under 6.0 percent and in excessof 15.0 percent, whereas high fracture strength was obtained between 6.0and 15.0 percent.

FIG. 6 shows the relationship between boron content and the fracturestrength of bonded joints. The fracture strength of bonded jointslowered when boron content was under 0.1 percent because of the rise inthe melting point and over 5.0 percent because of the borides formednear the bonding interface. High fracture strength was obtained withboron contents between 0.5 and 5.0 percent.

FIG. 7 shows the relationship between the thickness of the insertedmetal and the bonding time required for obtaining a bonded joint havinga tensile strength of not lower than 40 kg/mm². The alloy foilsaccording to this invention proved to require impractically long bondingtimes of over 10 hours when thickness exceeds 300 μm.

Table 2 shows the chemical compositions and melting points of the alloyfoils prepared for the purpose of comparison with those according tothis invention, tensile fracture strengths of the bonded joints of theclad steels prepared by the same method as that used with the preferredembodiment of this invention, and the thickness D of the oxide filmsformed at the surface of the bonded joints when oxidized in theatmosphere by heating at 700° C. for 1000 hours as an index of oxidationresistance. With the oxidation resistance of ferritic heat-resistingsteels used at 500° to 650° C. in mind, the threshold value of thethickness D was set at 20 μm. Bonded joints are considered to maintainadequate oxidation resistance at high temperatures for a long time if Dis 20 μm or under. The thickness D of all of the alloy foils accordingto this invention satisfying the requirements of claims 1 to 5 listed inTable 1 was not greater than 20 μm.

Table 2 lists the alloy foils prepared for the purpose of comparison.Foil No. 66 exhibited a low fracture strength because of an insufficientboron content and a melting point above 1300° C. Foil No. 67 exhibited amarkedly low fracture strength because of a high boron content and largequantities of coarse borides formed in the bonded metals near the bondedjoint. Foils Nos. 68 and 69 exhibited low fracture strengths because ofan insufficient silicon content, with a melting point exceeding 1300°C., and because of an excessive silicon content, with coarse siliconoxide-base oxides formed in the inserted metal. Foil No. 70 had adetrimental chromium oxide film formed at the surface of the bondedmetal due to vanadium deficiency. Foil No. 71 failed to produce thoroughliquid-phase diffusion bonding because of a vanadium content exceeding10.0 percent and an extremely high melting point. Foil No. 75 exhibiteda poor oxidation resistance due to chromium deficiency. Foil No. 73exhibited a poor oxidation resistance due to molybdenum deficiency.Foils Nos. 74 and 75 exhibited insufficient bonded joint strengths dueto niobium and tungsten deficiencies. Foil No. 76 exhibited aninsufficient bond strength due to an excess of tungsten and a meltingpoint exceeding 1300° C. Foil No. 77 exhibited a poor bond strengthbecause of an excess of nitrogen that formed coarse precipitates ofboron nitride with a high melting point, in conjunction with thediffusing atoms of boron.

                  TABLE 1-1-1                                                     ______________________________________                                        Foils of This Invention                                                       Chemical Compositions (in mass percent)                                       No.  Si      Mn     Cr   Mo    V    Nb   W     N                              ______________________________________                                        1    6.25    1.26   28.96                                                                              4.86  4.201                                                                              0.852                                                                              4.30  0.400                          2    7.26    0.33   2.41 3.07  2.887                                                                              0.394                                                                              1.33  0.251                          3    9.77    1.01   6.27 2.33  0.601                                                                              0.995                                                                              2.14  0.085                          4    7.89    0.17   21.51                                                                              1.93  0.719                                                                              0.555                                                                              3.52  0.062                          5    13.62   0.33   28.19                                                                              0.98  1.536                                                                              0.856                                                                              4.61  0.348                          6    11.96   1.18   6.74 1.20  1.603                                                                              0.276                                                                              2.19  0.127                          7    6.05    0.53   26.41                                                                              1.77  4.075                                                                              0.622                                                                              0.40  0.304                          8    12.32   1.84   8.26 1.89  2.681                                                                              0.745                                                                              3.30  0.304                          9    6.78    1.02   21.31                                                                              0.72  5.287                                                                              0.481                                                                              2.71  0.046                          10   10.51   0.61   16.71                                                                              4.85  5.622                                                                              0.652                                                                              0.30  0.430                          11   8.43    0.27   3.78 3.63  2.165                                                                              0.613                                                                              0.18  0.153                          12   8.25    0.66   7.78 3.64  2.782                                                                              0.189                                                                              3.94  0.110                          13   12.73   1.46   23.39                                                                              4.07  5.953                                                                              0.706                                                                              3.51  0.472                          14   7.19    1.43   16.86                                                                              1.69  5.443                                                                              0.530                                                                              2.96  0.422                          15   8.00    0.24   27.12                                                                              2.99  4.294                                                                              0.954                                                                              3.86  0.115                          16   13.55   1.35   19.74                                                                              4.75  5.736                                                                              0.853                                                                              0.58  0.465                          17   9.11    0.70   5.07 3.72  8.358                                                                              0.066                                                                              2.13  0.404                          18   6.56    0.80   22.07                                                                              1.51  0.880                                                                              0.385                                                                              2.75  0.339                          19   13.47   0.39   7.35 2.27  0.927                                                                              0.186                                                                              4.13  0.443                          20   14.88   1.08   29.61                                                                              2.73  2.117                                                                              0.771                                                                              2.98  0.385                          21   13.79   0.63   10.24                                                                              4.52  7.642                                                                              0.520                                                                              3.46  0.332                          22   13.22   1.59   18.16                                                                              4.40  1.656                                                                              0.235                                                                              2.19  0.440                          23   7.12    0.78   18.68                                                                              3.94  1.600                                                                              0.749                                                                              3.40  0.397                          24   7.66    0.77   22.29                                                                              2.45  1.240                                                                              0.427                                                                              1.32  0.113                          25   8.76    1.23   24.03                                                                              1.55  8.480                                                                              0.580                                                                              2.51  0.492                          26   13.47   1.86   18.96                                                                              2.86  2.836                                                                              0.073                                                                              1.30  0.476                          27   11.39   0.44   2.33 3.16  1.674                                                                              0.699                                                                              1.75  0.471                          28   9.93    1.41   13.35                                                                              4.03  3.326                                                                              0.814                                                                              3.01  0.146                          29   7.62    0.65   17.65                                                                              4.70  6.418                                                                              0.769                                                                              4.33  0.408                          30   12.03   0.70   10.52                                                                              1.92  4.530                                                                              0.389                                                                              2.48  0.442                          31   12.66   1.25   16.63                                                                              4.71  0.928                                                                              0.989                                                                              1.48  0.407                          32   12.55   1.05   16.37                                                                              4.87  8.229                                                                              0.941                                                                              0.63  0.477                          33   10.25   0.73   5.03 4.09  1.198                                                                              0.570                                                                              3.95  0.071                          ______________________________________                                    

                  TABLE 1-1-2                                                     ______________________________________                                        Foils of This Invention                                                       Chemical Compositions (in mass percent)                                       No.  B       C      Ti   Zr  MP    RS   TH   METHOD                           ______________________________________                                        1    4.621   --     --   --  1088  71   194  2                                2    0.688   --     --   --  998   58   201  1                                3    4.809   --     --   --  912   63   74   1                                4    3.931   --     --   --  1020  60   105  2                                5    2.149   --     --   --  1066  65   139  1                                6    3.561   --     --   --  928   63   16   1                                7    4.930   --     --   --  1034  60   34   1                                8    0.534   --     --   --  1008  69   32   2                                9    1.800   --     --   --  1080  64   5    2                                10   0.593   --     --   --  1073  63   242  1                                11   1.029   --     --   --  986   57   144  2                                12   4.676   --     --   --  956   63   293  1                                13   3.516   --     --   --  1051  71   242  2                                14   1.567   --     --   --  1068  67   281  1                                15   2.849   --     --   --  1094  64   100  2                                16   4.242   --     --   --  1001  68   294  1                                17   2.435   --     --   --  1002  62   256  2                                18   1.485   --     --   --  1073  63   278  1                                19   3.116   --     --   --  944   62   57   2                                20   4.034   --     --   --  1032  68   31   2                                21   3.456   --     --   --  991   66   44   2                                22   0.482   --     --   --  1056  68   238  1                                23   4.242   --     --   --  1018  65   161  2                                24   4.054   --     --   --  1017  61   249  2                                25   1.586   --     --   --  1107  67   55   1                                26   0.970   --     --   --  1043  68   160  1                                27   1.346   --     --   --  961   61   265  2                                28   1.909   --     --   --  1031  68   211  2                                29   1.875   --     --   --  1090  67   287  1                                30   2.910   --     --   --  979   63   44   2                                31   4.151   0.964  --   --  953   68   216  2                                32   4.891   0.565  --   --  979   68   202  2                                33   1.780   0.072  --   --  983   64   292  1                                ______________________________________                                         MP: Melting point of inserted metal (°C.)                              RS: Tensile strength of liquidphase diffusion bonded joint (kg/mm.sup.2)      TH: Thickness of inserted metal (μm)                                       METHOD: Methods of preparing inserted metal (1: quenching with single         roll; 2: hotrolling cast alloy prepared in vacuum melting furnace)       

                  TABLE 1-2-1                                                     ______________________________________                                        Foils of This Invention                                                       Chemical Compositions (in mass percent)                                       No.  Si      Mn     Cr    Mo   V    Nb    W    N                              ______________________________________                                        34   8.75    1.88   21.90 1.55 1.953                                                                              0.203 0.84 0.094                          35   11.01   1.33   17.66 3.75 4.222                                                                              0.662 2.75 0.467                          36   11.15   0.67   2.56  0.14 1.697                                                                              0.316 1.55 0.371                          37   7.73    0.57   1.38  2.20 2.356                                                                              0.479 1.33 0.464                          38   12.64   1.61   8.71  1.65 9.279                                                                              0.978 3.70 0.169                          39   10.94   1.62   20.17 2.73 6.806                                                                              0.197 0.56 0.150                          40   10.28   0.29   16.27 4.58 2.804                                                                              0.909 0.89 0.320                          41   13.48   0.12   6.25  3.87 5.104                                                                              0.491 3.82 0.354                          42   10.20   0.84   0.81  4.82 7.657                                                                              0.768 1.32 0.179                          43   7.94    1.16   24.05 4.08 8.643                                                                              0.084 2.27 0.438                          44   9.75    0.81   27.44 0.92 6.691                                                                              0.747 1.05 0.325                          45   9.62    1.33   5.69  4.31 9.630                                                                              0.134 1.69 0.243                          46   10.43   1.45   27.12 3.54 1.540                                                                              0.033 0.92 0.355                          47   6.42    1.86   29.29 2.90 9.734                                                                              0.319 0.52 0.348                          48   9.61    1.28   21.30 1.47 3.856                                                                              0.810 2.04 0.378                          49   9.38    1.29   27.92 0.92 1.422                                                                              0.363 2.22 0.364                          50   8.90    0.64   14.93 2.91 0.873                                                                              0.780 2.72 0.228                          51   6.58    1.84   6.64  4.13 0.614                                                                              0.427 2.72 0.134                          52   14.49   0.81   28.06 1.39 8.698                                                                              0.624 1.47 0.248                          53   13.14   1.02   0.60  1.22 9.241                                                                              0.710 3.01 0.241                          54   10.07   0.27   22.43 0.35 5.398                                                                              0.530 1.68 0.036                          55   12.82   1.09   29.95 2.89 5.184                                                                              0.902 4.00 0.042                          56   10.85   1.27   18.09 3.12 6.967                                                                              0.675 4.23 0.173                          57   7.27    1.44   11.63 2.69 9.188                                                                              0.681 4.07 0.356                          58   14.01   0.36   5.83  3.27 0.937                                                                              0.759 2.73 0.020                          59   7.17    0.56   25.25 1.59 9.007                                                                              0.748 4.29 0.064                          60   12.24   0.77   20.67 1.71 3.302                                                                              0.963 0.52 0.156                          61   14.22   0.67   2.13  2.89 1.411                                                                              0.277 0.50 0.221                          62   7.11    0.90   8.92  4.26 1.629                                                                              0.486 0.96 0.182                          63   7.53    1.18   7.04  2.64 7.258                                                                              0.445 3.52 0.439                          64   13.84   1.14   20.47 2.02 7.128                                                                              0.448 2.49 0.187                          65   11.32   0.54   1.04  4.18 4.615                                                                              0.461 2.00 0.076                          ______________________________________                                    

                  TABLE 1-2-2                                                     ______________________________________                                        Foils of This Invention                                                       Chemical Compositions (in mass percent)                                       No.  B       C      Ti    Zr   MP   RS   TH   METHOD                          ______________________________________                                        34   3.833   0.807  --    --   995  67   184  2                               35   2.954   0.627  --    --   1015 69   17   1                               36   0.750   --     3.313 --   976  60   82   2                               37   3.768   --     4.274 --   944  60   77   2                               38   3.797   --     0.615 --   981  70   141  1                               39   2.615   --     4.558 --   1067 67   116  2                               40   1.756   --     0.781 --   1040 61   294  1                               41   2.883   --     --    4.474                                                                              993  63   75   2                               42   0.351   --     --    1.914                                                                              1025 64   47   2                               43   0.648   --     --    1.205                                                                              1149 67   55   1                               44   3.008   --     --    2.055                                                                              1082 64   93   1                               45   1.651   --     --    4.171                                                                              1046 67   299  2                               46   3.181   --     0.560 2.232                                                                              1064 66   74   2                               47   4.401   --     3.874 3.411                                                                              1129 70   300  1                               48   4.430   --     1.963 4.902                                                                              1041 68   82   2                               49   4.934   --     1.982 1.382                                                                              1034 66   153  2                               50   0.209   --     1.142 2.540                                                                              1077 64   211  2                               51   1.701   0.391  1.647 --   1004 68   248  2                               52   3.725   0.428  0.669 --   1046 66   257  2                               53   2.989   0.083  0.393 --   945  65   230  2                               54   3.628   0.204  2.981 --   1037 60   163  1                               55   2.367   0.619  3.152 --   1102 71   47   2                               56   3.266   0.167  --    3.004                                                                              1054 70   172  2                               57   3.128   0.678  --    0.746                                                                              1029 70   84   2                               58   1.148   0.998  --    0.671                                                                              954  62   77   2                               59   1.405   0.462  --    4.942                                                                              1152 67   140  1                               60   0.623   0.893  --    0.773                                                                              1043 64   269  2                               61   0.451   0.636  3.325 2.840                                                                              973  61   157  2                               62   0.122   0.529  2.809 3.387                                                                              1061 63   243  2                               63   1.243   0.896  4.846 4.537                                                                              1069 69   79   1                               64   3.031   0.095  1.678 3.522                                                                              1055 68   99   2                               65   2.270   0.449  2.915 2.449                                                                              975  63   151  2                               ______________________________________                                         MP: Melting point of inserted metal (°C.)                              RS: Tensile strength of liquidphase diffusion bonded joint (kg/mm.sup.2)      TH: Thickness of inserted metal (μm)                                       METHOD: Methods of preparing inserted metal (1: quenching with single         roll; 2: hotrolling cast alloy prepared in vacuum melting furnace)       

                                      TABLE 2-1                                   __________________________________________________________________________    Alloy Foils for Comparison                                                    Chemical Composition (in mass percent)                                        Si   Mn Cr Mo V  Nb W  N  B  C  Ti Zr                                         __________________________________________________________________________    66                                                                              7.3                                                                              0.51                                                                             12.1                                                                             0.36                                                                             4.12                                                                             0.041                                                                            1.063                                                                            0.062                                                                            0.613                                                                            0.044                                            67                                                                              9.1                                                                              0.52                                                                             11.5                                                                             0.25                                                                             4.23                                                                             0.043                                                                            1.002                                                                            0.041                                                                            7.161                                                                            0.211                                                                            0.225                                         68                                                                              4.3                                                                              0.51                                                                             10.1                                                                             0.25                                                                             2.00                                                                             0.051                                                                            1.515                                                                            0.047                                                                            2.240                                                                            0.056 0.120                                      69                                                                              1.1                                                                              0.50                                                                             15.6                                                                             0.51                                                                             2.60                                                                             0.028                                                                            1.800                                                                            0.022                                                                            3.114                                               70                                                                              12.0                                                                             0.49                                                                             9.8                                                                              0.66                                                                             0.41                                                                             0.060                                                                            1.916                                                                            0.044                                                                            4.180                                                                            0.335                                                                            0.616                                         71                                                                              11.0                                                                             0.48                                                                             0.26                                                                             0.42                                                                             11.6                                                                             0.150                                                                            1.212                                                                            0.069                                                                            3.552                                                                            0.841 0.012                                      72                                                                              9.1                                                                              0.48                                                                             9.0                                                                              4.40                                                                             5.12                                                                             0.111                                                                            0.557                                                                            0.071                                                                            2.206                                                                            0.090                                            73                                                                              8.5                                                                              0.49                                                                             9.0                                                                              0.06                                                                             9.96                                                                             0.533                                                                            2.262                                                                            0.102                                                                            4.115                                                                            0.064                                            74                                                                              8.6                                                                              0.49                                                                             8.9                                                                              2.10                                                                             9.89                                                                             0.004                                                                            3.334                                                                            0.115                                                                            4.028 0.709                                         75                                                                              9.2                                                                              0.53                                                                             25.1                                                                             0.64                                                                             1.00                                                                             0.244                                                                            0.065                                                                            0.332                                                                            2.186    3.96                                       76                                                                              7.7                                                                              0.68                                                                             2.24                                                                             0.67                                                                             7.67                                                                             0.816                                                                            4.419                                                                            0.229                                                                            0.595                                                                            0.091                                                                            4.15                                                                             0.151                                      77                                                                              7.9                                                                              0.52                                                                             25.0                                                                             1.05                                                                             0.99                                                                             0.091                                                                            0.222                                                                            0.770                                                                            0.270                                                                            0.118                                            __________________________________________________________________________

                                      TABLE 2-2                                   __________________________________________________________________________    Alloy Foils for Comparison                                                    Melting Point of                                                                        Tensile Strength                                                                      Thickness of                                                                         Thickness of                                                                          Method of                                    Inserted Metal                                                                          of Bonded Joint                                                                       Oxide Film D                                                                         Inserted Metal d                                                                      Preparing                                    (°C.)                                                                            (kg/mm.sup.2)                                                                         (μm)                                                                              (μm) Inserted Metal                               __________________________________________________________________________    66                                                                              1320    16.1    18     33      1                                            67                                                                              960     8.6     16     71      1                                            68                                                                              1360    12.0    71     42      1                                            69                                                                              1000    10.5    15     58      2                                            70                                                                              990     6.2     15     60      2                                            71                                                                              1430    3.5     19     32      2                                            72                                                                              1040    42.0    460    30      1                                            73                                                                              980     39.2    330    102     2                                            74                                                                              980     22.5    8      150     2                                            75                                                                              1070    26.1    6      120     1                                            76                                                                              1430    8.5     15     100     1                                            77                                                                              1160    11.4    14     41      1                                            __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                        Chemical Compositions of Bonded Metals (in mass percent)                      C       Si      Mn     Cr    Mo   Ni    W    Co                               ______________________________________                                        A   0.225   0.310   0.506                                                                              2.24  1.06 0.21  1.01 3.15                           B   0.086   0.064   0.510                                                                              8.86  0.54 0.56  1.96 --                             C   0.164   0.209   0.766                                                                              11.77 1.25 --    2.26 2.50                           D   0.033   0.150   4.20 19.06 2.25 26.0  2.66 2.01                           E   0.006   1.525   16.75                                                                              16.33 4.05 8.11  --   --                             F   0.020   0.250   0.505                                                                              28.16 3.55 62.69 --   --                             ______________________________________                                    

Industrial Advantages of the Invention

As described above, this invention provides alloy foils for liquid-phasediffusion bonding in oxidizing atmospheres that produce bonded jointswith extremely high fracture strength between heat-resisting metals,thus making a valuable contribution to the development of industries.

What is claimed is:
 1. Alloy foil for liquid-phase diffusion bonding ofheat-resisting metals in an oxidizing atmosphere consisting essentiallyof 6.0 to 15.0 percent silicon, 0.1 to 2.0 percent manganese, 0.50 to30.0 percent chromium, 0.10 to 5.0 percent molybdenum, 0.50 to 10.0percent vanadium, 0.02 to 1.0 percent niobium, 0.10 to 5.0 percenttungsten, 0.01 to 0.5 percent nitrogen, 0.10 to 5.0 percent boron, inpercent by mass, and the balance nickel and impurities, and having athickness of 3.0 to 300 μm.
 2. Alloy foil for liquid-phase diffusionbonding of heat-resisting metals in an oxidizing atmosphere consistingessentially of 6.0 to 15.0 percent silicon, 0.1 to 2.0 percentmanganese, 0.50 to 30.0 percent chromium, 0.10 to 5.0 percentmolybdenum, 0.50 to 10.0 percent vanadium, 0.02 to 1.0 percent niobium,0.10 to 5.0 percent tungsten, 0.01 to 0.5 percent nitrogen, 0.10 to 5.0percent boron, 0.005 to 1.0 percent carbon, in percent by mass, and thebalance nickel and impurities, and having a thickness of 3.0 to 300 μm.3. Alloy foil for liquid-phase diffusion bonding of heat-resistingmetals in an oxidizing atmosphere consisting essentially of 6.0 to 15.0percent silicon, 0.1 to 2.0 percent manganese, 0.50 to 30.0 percentchromium, 0.10 to 5.0 percent molybdenum, 0.50 to 10.0 percent vanadium,0.02 to 1.0 percent niobium, 0.10 to 5.0 percent tungsten, 0.01 to 0.5percent nitrogen, 0.10 to 5.0 percent boron, at least one selected fromthe group consisting of 0.01 to 5.0 percent titanium and 0.01 to 5.0percent zirconium, in percent by mass, and the balance nickel andimpurities, and having a thickness of 3.0 to 300 μm.
 4. Alloy foil forliquid-phase diffusion bonding of heat-resisting metals in an oxidizingatmosphere consisting essentially of 6.0 to 15.0 percent silicon, 0.1 to2.0 percent manganese, 0.50 to 30.0 percent chromium, 0.10 to 5.0percent molybdenum, 0.50 to 10.0 percent vanadium, 0.02 to 1.0 percentniobium, 0.10 to 5.0 percent tungsten, 0.01 to 0.5 percent nitrogen,0.10 to 5.0 percent boron, 0.005 to 1.0 percent carbon, at least oneselected from the group consisting of 0.01 to 5.0 percent titanium and0.01 to 5.0 percent zirconium, in percent by mass, and the balancenickel and impurities, and having a thickness of 3.0 to 300 μm.
 5. Alloyfoil for liquid-phase diffusion bonding of heat-resisting metals in anoxidizing atmosphere according to claim 1, that has a substantiallyvitreous structure.
 6. Alloy foil for liquid-phase diffusion bonding ofheat-resisting metals in an oxidizing atmosphere according to claim 2that has a substantially vitreous structure.
 7. Alloy foil forliquid-phase diffusion bonding of heat-resisting metals in an oxidizingatmosphere according to claim 3 that has a substantially vitreousstructure.
 8. Alloy foil for liquid-phase diffusion bonding ofheat-resisting metals in an oxidizing atmosphere according to claim 4that have a substantially vitreous structure.